In conventional automatic transmissions, one frictional engagement element and a one-way clutch are generally engaged to establish a starting gear position. However, such a one-way clutch is heavy, and may cause drag in gear positions except for the starting gear position. Accordingly, an attempt has been made to eliminate such a one-way clutch for, e.g., improving engine fuel consumption performance, and to engage two frictional engagement elements to establish the starting gear position.
Vehicles equipped with an idle stop control system have been commercially practical, in which an engine is automatically stopped when a predetermined stop condition is satisfied. Automatic transmissions mounted in such vehicles are provided with an electrically driven oil pump (hereinafter referred to as “electric pump”) in addition to a mechanically driven oil pump (hereinafter referred to as “mechanical pump”) driven by an engine to enable quick start of the vehicle in the next start. Supplying a hydraulic pressure generated by this electric pump allows for, even during an engine stop, advance engagement of start frictional engagement elements which transmit a motive force in starting the vehicle.
However, there may be a situation where a vehicle's operator allows an automatic transmission to switch from a traveling range such as D-range to a non-traveling range such as N-range before or after the engine is automatically stopped when the vehicle is stopped. In such a situation, an engagement hydraulic pressure is released from each of the frictional engagement elements in this automatic transmission. Thus, the start frictional engagement elements are not engaged even if the electric pump is driven while the engine is being automatically stopped. The start frictional engagement elements will be engaged again when a switch is made to the traveling range in starting the vehicle. At that time, however, the vehicle may possibly fail to start smoothly after a restart of the engine due to engagement delay of the frictional engagement elements.
In order to handle such a problem, Patent Document 1 discloses an automatic transmission including an electric pump which is operated when the engine has been automatically stopped. This automatic transmission is allowed to control supply of a hydraulic pressure generated by the electric pump to start frictional engagement elements even when a switch is made to a non-traveling range in the automatic transmission before or after an automatic stop of the engine, thereby allowing for engaging the start frictional engagement elements in advance in this non-traveling range just like in a traveling range. According to this configuration, when a switch is made to the traveling range in the automatic transmission in starting the vehicle, the vehicle can be started immediately after a restart of the engine. This configuration allows for a smooth start-up of the vehicle.